Watanabe et al. [1] (hereafter referred to as WMHY) used A-bomb survivor data from the Radiation Effects Research Foundation (RERF) Life Span Study (LSS) Report 12 (available online at http://www.rerf.or.jp) and mortality rates for Hiroshima and Okayama Prefectures [2] to create standardized mortality ratios (SMR) for LSS cohort members residing in Hiroshima at the time of the bombing. WMHY divided cohort members into three radiation dose categories: very low dose (\0.005 Gy, VLD), low dose (0.005-0.1 Gy, LD), and high dose ([0.1 Gy, HD). At issue are the men in the VLD category, who experienced higher than expected deaths due to cancers compared with prefectural rates (note that, for brevity, we discuss only the solid cancer results). WMHY concluded that the increased cancer deaths among the VLD were due to underestimated neutron doses and/or unaccounted-for residual radiation exposures. We find their conclusions to be implausible and believe that the data show it is far more likely that the observed risks among the VLD are due to nonradiation factors. The reasons are briefly summarized below.
Implausible sex-specific risksIf there were unaccounted-for radiation exposures, it is reasonable that they would have exposed men and women alike. However, the SMR values for VLD women were very close to 1.0; only those for men were elevated. In a related observation, previous studies have shown that the excess relative risk (ERR) per gray (Gy) of cancer mortality after radiation exposure is consistently higher for women compared with men [3,4]. Thus, if radiation were responsible for the high SMR levels, we would expect to see higher SMRs in women compared with men. Using the estimates from Table 1 in WMHY [1], the sex ratio of excess risks for solid cancer is greater than unity in the HD group (female/male = 0.64/0.41 = 1.56) as expected, but is less than unity for the LD group (0.10/0.20 = 0.50) and even smaller for the VLD group (0.04/0.18 = 0.22). This pattern supports the conclusion that the dominant cause for the high solid cancer SMR observed in the HD group is radiation, whereas, in the LD and VLD groups, the elevated SMRs are primarily due to nonradiation factors.
Implausibly large effectsIf the elevated SMR value for the VLD male survivors was indeed due to radiation, the average acute dose level required to produce the observed SMR can be estimated by dividing the excess solid cancer mortality ratio for the VLD males (0.18) by the ERR/Gy estimate for males in the LSS cohort (0.38 [3]), giving 0.18/0.38 Gy -1 & 0.5 Gy. This is a high level of exposure that was experienced by only about 7% of the LSS cohort, located at distances less than about 1,600 m from the hypocenter. Moreover, if a plume of radioactive materials were to have become airborne and